What does it do?

Adds, multiplies the currency amounts, and calculates percentages of amounts. The result of
each of those operations is also an amount: a string, strictly matching the /^\-?\d+\.\d\d$/
pattern, like "0.25", "1000.00", or "-42.10".

Which we use for bills in CHF that are required by law to be 0 (mod 5).

An important note on the amount "data type"

The amount strings are expected to strictly adhere to the format described by the regular
expression noted above. Thus, for example, it must be:

"10.10", not "10.1", not "10.100";

"10.00", not 10, not "10", not "10.0".

That's a precondition for any of the API functions accepting amount arguments to work correctly. I
understand that it may be confusing to some of new users; but I believe that's an optimally
pragmatic way to mimic, by convention, an algebraic data type in idiomatic JavaScript -- a (very)
dynamically typed language.

Luckily, you can always move your arbitrary float value into the amounts field with
money.floatToAmount(...). Once all the values are amounts, money-math guarantees that all the
field operations keep the results withing the field. Classic algebra.

A thoughtful reader may ask, why have money.floatToAmount(), when there's the
Number.prototype.toFixed(2)? Well, because: